The invention relates to a composite sheet having local thickness variations generally obtained by localized adhesive bonding of reinforcement sheet blanks or xe2x80x9cpatchesxe2x80x9d to a main sheet of uniform thickness; such a composite sheet is often called a xe2x80x9cpatchwork sheetxe2x80x9d.
This type of composite sheet makes it possible to lighten motor-vehicle structures since only the regions most stressed are provided with adhesively bonded reinforcement blanks, that is to say the regions are xe2x80x9cpatchedxe2x80x9d; the composite sheet then has the smaller thickness in the unreinforced regions and the larger thickness in the reinforced regions.
These composite sheets must be able to be drawn, like conventional sheets of uniform thickness, so as, in particular, to produce motor-vehicle structural or body parts; the major problem is then the ability of the adhesive joints, which join the xe2x80x9cpatchesxe2x80x9d to the sheet, to withstand the stresses and strains applied during the drawing operation.
Documents DE, 4,307,563 (BMW) and DE 19,524,235 (VW) describe composite sheets of this type, comprising a main sheet locally reinforced by adhesively bonded patches in the regions to be reinforced. The patches may have different thicknesses and mechanical properties, tailored to the reinforcement requirements specific to each region to be reinforced.
Adhesives envisaged for producing this type of sheet are, for example, of the epoxy type; unfortunately, as the results presented below confirm, the composite sheets obtained are unable to withstand drawing operations well, especially because:
the adhesive joints are not generally strong enough to withstand the deformation stresses applied during the forming operation;
the adhesive joints are generally not ductile enough to deform with the sheet elements during the forming operation.
To avoid these drawbacks, DE 4,307,563 and EP 823,297 describe a process for manufacturing formed parts made of composite sheet of this type, such as motor-vehicle body parts, in which:
before the forming, the consolidation of the various sheet elements of the composite sheet is only partial (the xe2x80x9ctackyxe2x80x9d effect of the adhesive is used) so that the adhesive joint between these elements can undergo, without damage, the deformations applied during the forming operation;
after the forming, the consolidation of the various elements of the composite sheet is reinforced by crosslinking the adhesive joints, which no longer have to undergo deformation.
This process exhibits certain drawbacks:
it is limited to bonding systems based on a crosslinkable adhesive, thereby excluding thermoplastic adhesives;
before forming, the composite sheet must be handled with care, in order to prevent any risk of changing the position of the xe2x80x9cpatchesxe2x80x9d;
after forming, it is necessary to add a curing step which is economically penalizing.
The object of the invention is to avoid the aforementioned drawbacks by providing a system for adhesively bonding the elements of the composite sheet which, although completely activated before the forming, possesses the intrinsic mechanical properties required so as not to be degraded during a drawing-type forming operation.
For this purpose, the subject of the invention is a composite sheet intended for drawing, comprising at least one main sheet and at least one patching sheet blank adhesively bonded as a patch to a region of the said main sheet to be reinforced by means of a bonding system, characterized in that the said bonding system is chosen, applied and, where appropriate, crosslinked so as to satisfy the following intrinsic criteria within the temperature range of the drawing operation to which it is intended:
a critical energy release rate in mode I (tension) greater than 1500 J/m2;
a critical energy release rate in mode II (shear) greater than 2000 J/m2.
The statement whereby the said bonding system is, where appropriate, crosslinked means that if this bonding system comprises crosslinkable components, the system for bonding the composite sheet is actually crosslinked.
The said temperature range corresponds to the temperature that the composite sheet reaches during the drawing operation for which it is intended, more specifically to the temperature of the bonding system for this sheet; under standard conditions, this temperature range generally corresponds to the 15xc2x0 C.-40xc2x0 C. interval; the critical energy release criteria are therefore considered in this range.
The invention may also exhibit one or more of the following features;
the said bonding system is chosen so as to furthermore satisfy the following relative criterion: an elongation at break greater than that of the metal of the said main sheet and greater than that of the metal of the said patch blank, preferably by more than 40%;
the said bonding system comprises one or more thermoplastic and/or thermosetting and/or elastomeric resins, which may or may not be linked by physical and/or chemical bonds;
the thickness of the said bonding system is between 0.05 and 5 mm;
the said bonding system comprises a polymer material chosen from the group comprising polyolefins and polyvinyl chlorides;
the said bonding system comprises a polyolefin polymer material characterized by a melting point of greater than 120xc2x0 C.;
the said bonding system comprises a polyolefin polymer material characterized by a melting point of greater than 160xc2x0 C.;
the said bonding system comprises a core made of a polymer material and two thin layers of adhesive, one being inserted between the said main sheet and one face of the core, the other being inserted between the said patching sheet and the other face of the core; preferably, the thickness of each of the said thin layers is then between 5 and 50 xcexcm.
The subject of the invention is also a process for manufacturing a formed part using a composite sheet previously described, characterized in that it comprises the steps in which:
the said main sheet and at least one patching blank are prepared;
then, by inserting the bonding system, the said patching blank is applied to that region of the said main sheet to be reinforced;
then, where appropriate, the said bonding system is crosslinked so as to obtain the said composite sheet; and
then the said composite sheet obtained is formed by drawing.
The statement according to which xe2x80x9cthe said bonding system is, where appropriate, crosslinkedxe2x80x9d means that if this bonding system comprises crosslinkable components, the bonding system for the composite sheet is actually crosslinked before the forming.
Thus, formed parts are obtained which are effectively reinforced without having to necessarily employ a curing step after forming.